In recent years, more and more portable or cordless household electronic appliances have been used. With reduction in size and weight of electronic appliances, nonaqueous electrolyte secondary batteries such as nickel cadmium battery, nickel-metal hydride battery, small sealed lead acid storage battery and the like have come into use as a power supply for driving electric appliances. In a nonaqueous electrolyte secondary battery, in order to achieve highly efficient charge and discharge performance, a positive electrode plate and a negative electrode plate are spirally wound with a separator interposed therebetween to form an electrode group in general.
As the size and weight of electronic appliances have been reduced, in view of effectively utilizing a space of an electronic appliance, a rectangular flat shape has been adopted for secondary batteries and lightweight alloy such as aluminum and the like has been used as a material for armoring cans.
However, when a charge and discharge cycle of a battery is repeated, an internal pressure is applied to an armoring can due to swelling of an electrode group or the like. In the case of a cylindrical battery, an internal pressure is uniformly applied to an entire side of an armoring can. In the case of a rectangular flat battery, however, a problem arises in which pressure-resisting strength of an armoring can is low at its long side surface, so that the long side surface swells.
To cope with this problem, in Patent Reference 1, a technique in which a concave portion is formed in a long side surface of an armoring can is proposed. Specifically, by forming a concave portion in a long side surface of an armoring can, an internal pressure applied to the long side surface is uniformed and thus swelling in the long side surface of the armoring can prevented.
However, swelling of an electrode group occurring due to repetition of a charge and discharge cycle of a battery causes another problem, i.e., internal resistance of the battery is increased. It is considered that this problem arises because a void (winding core hole) is left in a center portion of the electrode group after removing a winding core and part of the electrode plates deflects due to swelling of the electrode group, so that a distance between electrode plates is increased.
To solve the above-described problem, in Patent Reference 2, a method in which an elastic body is inserted in a winding core hole after removal of a winding core is proposed. Specifically, the elastic body inserted in the winding core hole applies a pressure to an electrode group to prevent deflection of the electrode group toward the winding core hole is prevented, so that increase in internal resistance is prevented. The member inserted in the winding core hole is an elastic body and the elastic body has the function of absorbing swelling of the electrode group. Therefore, swelling of an armoring can be prevented as well.    Patent Reference 1: Japanese Laid-Open Publication No. 2002-042741    Patent Reference 2: Japanese Laid-Open Publication No. H8-055637